ACTH stimulates steroidogenesis at a site between cholesterol and pregnenolone, a series of reactions which take place in adrenal mitochondria. These reactions are inhibited by the end product, pregnenolone at the first reaction of the sequence. The inhibition appears to be important in the control of pregnenolone synthesis since modifications of the membrane characteristics of mitochondria result in increased syntheses of pregnenolone, presumably, as a consequence of an increased efflux of mitochondrial pregnenolone with the resultant release of the feedback inhibition. The kinetic characteristics of pregnenolone formation are consistent with this model. A major prediction of the model is that the intra-mitochondrial concentration of pregnenolone will be decreased by ACTH. This is now one of our major efforts. Protein synthesis has long been implicated in ACTH action. Present evidence indicates that ACTH does not stimulate protein synthesis. It is consistent with the idea that ACTH mediated phosphorylation of a rapidly turning-over protein is involved. We are concerned with getting data on this hypothesis and the isolation of the active protein.